TUESDAY, July 31 (HealthDay News) -- A change in vaccine composition may partly explain current whooping cough outbreaks in the United States and Australia, researchers say.

In the 1990s, a switch was made from diphtheria-tetanus-whole cell pertussis (DTwP) vaccine to diphtheria-tetanus-acellular pertussis (DTaP) vaccine to reduce side effects. It's possible that the newer vaccine is less effective and wears off faster, Australian researchers suggest.

"Our findings don't change the fact that vaccinations remain the best way to prevent whooping cough," said lead researcher Stephen Lambert, an associate professor at the Queensland Children's Medical Research Institute of the University of Queensland in Brisbane.

"Children who develop pertussis [whooping cough] despite being vaccinated have milder symptoms, reduced duration of illness and are less infectious to others than children who have not received their vaccines," he added.

Moreover, doctors shouldn't exclude whooping cough as a diagnosis just because a child has had all of the recommended vaccines, Lambert said.

Whooping cough -- so-called because of the distinctive crowing sound or "whoop" it causes -- is highly contagious. More than twice as many cases have been reported in the United States so far this year compared to last year, and already nine babies have died, according to the U.S. Centers for Disease Control and Prevention. "We may be on track for a record-high pertussis rate this year," Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said recently.

For the study, published in the Aug. 1 issue of the Journal of the American Medical Association, Lambert's team collected data on more than 58,000 Australian children born in 1998. More than 40,000 of them received three doses of a pertussis vaccine.

Children who received the DTaP vaccine had higher rates of whooping cough than those who got the DTwP vaccine, the investigators found.

Among those who received mixed vaccines, the rates of whooping cough were highest among children receiving DTaP as their first dose, they noted.

"Our findings go some way to explaining why we are currently seeing a resurgence of pertussis in Australia and the U.S., and in particular why rates are higher in some age groups compared to others," Lambert said.

In switching to a different vaccine, "we may have traded off some of the protection whole-cell vaccines provided for the better safety profile, with many fewer adverse events, seen with acellular vaccines," Lambert suggested.

The earlier vaccine caused pain and swelling at the injection site. Also, rare adverse brain effects occurred in some children, although it's not known if the vaccine was the direct cause.

Dr. Thomas Clark, a medical epidemiologist at the CDC, said the acellular vaccine was introduced because of those concerns. "What we know now is that the whole-cell vaccines were probably a little better than the acellular vaccines," he said.

The open question is how long the vaccine remains effective, Clark added.

While most older children and adults have few problems with whooping cough, infants are at great risk because their airway is underdeveloped. Vaccination can't take place until a child is 1 year old, which is why the CDC urges pregnant women and anyone who comes into contact with infants to be vaccinated. For the first year of life, a mother's immunity protects her baby, Clark added.

The vaccine's immunity wears off over time so children should get a booster at age 10, and adults need one about every 10 years, Clark said.

So far this year almost 19,000 cases of pertussis have been reported in the United States. Washington state has been particularly hard hit. Noting that these are reported cases, Clark said there could actually be 1 million to 1.5 million people with pertussis in the United States. Most of these undiagnosed cases are mild, he noted.

Clark urges everyone, including adults, to get vaccinated. "It's the best protection against the disease," he said. "We used to have upwards of 10,000 deaths a year," he pointed out.

Vaccination rates among older children and adolescents are high, but the rate for adults is only about 7 percent, he said.